What are hosted filters (Microsoft, Google, etc.)?
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If you send email to any real volume of people, chances are a large chunk of your list is reading it in Gmail or Microsoft 365. Both of those inboxes come with built-in spam filtering baked right into the mailbox provider itself. That's what a hosted filter is.
Unlike older on-premise filters that a company would run on its own servers, hosted filters live entirely in the cloud and are managed by the provider. No hardware, no IT team configuring rulesets. The provider handles everything, and every user on that platform gets the same protection automatically.
Here's what makes them genuinely powerful from a senders' perspective: scale. Google filters hundreds of billions of messages every day across Gmail and Google Workspace. Microsoft does the same through Exchange Online Protection, which covers every Microsoft 365 mailbox. When a new spam pattern hits one user, the filter can update for everyone on the platform within minutes. That's a feedback loop no single company's on-premise server could ever match.
What are they actually checking? A lot. Sender reputation is near the top of the list, which means your sending IP, your domain, and how recipients have interacted with your emails historically. They also check content signals, authentication (SPF, DKIM, DMARC), engagement patterns, and whether your domain or IP shows up on any blocklists.
Yahoo Mail and Outlook.com (the consumer version) run their own hosted filters too. So does Zoho Mail. The underlying logic varies by provider, but the structure is the same: cloud-based, centrally managed, learning from aggregate data at scale.
As a sender, you don't get to configure any of this. That's the important trade-off. The provider controls the filter, and there's no dial to turn. What you can influence is your own behavior: clean sending practices, proper authentication, healthy engagement rates, and not sending to people who didn't ask to hear from you. Those are the inputs these filters actually reward.
If you're not sure whether your authentication setup looks right to Gmail or Microsoft, our free Email Header Analyzer can show you exactly what those filters are seeing when your email arrives. Or if something's broken and you need a second opinion, the SOS hotline is free.
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